An increasing number of light fixtures utilize LEDs as light sources due to their lower energy consumption, smaller size, improved robustness, and longer operational lifetime relative to conventional filament-based light sources. Conventional LEDs emit quasi-monochromatic radiation ranging from infrared through the visible spectrum to ultraviolet. Conventional LEDs emit light at a particular wavelength, ranging from, for example, red to blue or ultraviolet (UV) light. However, for purposes of general illumination, the relatively narrow spectral width of light emitted light by LEDs is generally converted to broad-spectrum white light.
Traditionally, there are two ways of obtaining white light from LEDs. One approach utilizes two or more LEDs operating at different wavelengths, where the different wavelengths are chosen such that their combination appears white to the human eye. For example, one may use LEDs emitting in the red, green, and blue wavelength ranges. Such an arrangement typically requires careful control of the operating currents of each LED, such that the resulting combination of wavelengths is stable over time and different operating conditions, for example temperature. Because the different LEDs may be formed from different materials, different operating parameters may be required for the different LEDs; this complicates the LED circuit design. Furthermore, such systems typically require some form of light combiner, diffuser or mixing chamber, so that the eye sees white light rather than the distinct colors of each of the different LEDs. Such light-mixing systems typically add cost and bulk to lighting systems and may reduce their efficiency.
White light may also be produced in LED-based systems for general illumination via the utilization of light-conversion materials such as phosphors, sometimes called phosphor-converted LEDs. For example, an LED combined with a wavelength-conversion element (WCE) generates white light by combining the short-wavelength radiant flux (e.g., blue light) emitted by the semiconductor LED with long-wavelength radiant flux (e.g., yellow light) emitted by, for example one or more phosphors within the WCE. The chromaticity (or color), color temperature, and color-rendering index are determined by the relative intensities of the component colors. For example, the light color may be adjusted from “warm white” with a correlated color temperature (CCT) of 2700 Kelvin or lower to “cool white” with a CCT of 6500 Kelvin or greater by varying the type or amount of phosphor material. White light may also be generated solely or substantially only by the light emitted by the one or more phosphor particles within the WCE. A WCE may also be referred to as a phosphor conversion element (PCE) or a phosphor.
PCEs may be positioned in contact with the LED die or positioned apart—that is, remotely from the LED; in this configuration, the PCE is called a “remote phosphor.” Both remote-phosphor and contact-phosphor configurations produce a non-uniform color distribution as a function of the emission angle from the LED-based illumination system, thereby reducing the quality of light and the suitability of the light source for lighting products. Non-uniformity of the angular color distribution from phosphor-converted LEDs (PCLED) results from differences in the angular intensity distributions of the LED light and the phosphor-converted light. For example, the LED typically exhibits a Lambertian luminous intensity distribution pattern, while emission from the phosphor typically exhibits a substantially isotropic luminous intensity distribution. In the case of a phosphor-converted white LED, the blue light emitted from an LED die has a non-isotropic color distribution (such as a Lambertian distribution) whereas light converted by a PCE with a yellow emission peak has a substantially isotropic color distribution. As a consequence, the chromaticity of the combined light varies with viewing angle, resulting in a non-uniform color distribution as seen by the human eye. For example, a phosphor-coated blue LED may be typically perceived as being cool white when viewed head-on, but warm white when viewed obliquely.
Lighting and illumination systems that include LEDs frequently suffer from the angularly dependent color non-uniformity of phosphor-converted LEDs. In order to mitigate the relatively poor angular color uniformity of conventional phosphor-converted LEDs, such illumination systems often require additional elements, such as diffusers, mixing chambers, or the like, to homogenize the color characteristics. Such homogenization often degrades the light-intensity distribution pattern, however, resulting in the need for secondary optics to re-establish the desired light-intensity distribution pattern. The addition of these elements typically requires undesirable additional space or volume, adds cost and expense, and reduces output efficiency.
Accordingly, there is a need for structures, systems and procedures enabling LED-based illumination systems to generate uniform color distribution of light and operate with high extraction efficiency while utilizing low-cost integration of phosphors with the LEDs.